Abstract The Protease Activated Receptor (PAR1) is the receptor of thrombin, the main serine protease of the coagulation cascade. PAR1 is expressed by platelets, endothelial cells and cells of the innate immune system and mediates platelet aggregation and secretion of pro-inflammatory cytokines. PAR1 is expressed by T lymphocytes, however its role in T cell function is not well defined. We have previously shown that human CD4 and CD8 T cells express PAR1. PAR1 expression was higher in CD8 than CD4 T cells and is associated with T cell differentiation. CD8 T cell Effector Memory and Terminal Effector Memory phenotypes have higher PAR1 expression than Naïve T cells. PAR1 activation by thrombin enhanced CD8 T cell function including chemokinesis and cytokine secretion. In the present study, we address the role of PAR1 signaling in CD8 T cell mediated cytotoxicity by the granule exocytosis pathway. We found that expression of PAR1 on CD8 T cells is associated with expression of the transcription factors T-bet and Eomes and effector molecules, granzyme A, granzyme B and perforin. PAR1 activation by thrombin enhanced TcR-induced degranulation in CD8 T cells. In addition, in a redirected killing assay, we show that granule exocytosis and cytotoxicity of target cells was efficiently blocked by PAR1 antagonist (SCH79797). Confocal microscopy of the CD8 T-APC conjugates revealed that PAR1 blockade inhibit actin polymerization and decreased the docking of the Microtubule Organizing Center (MTOC) at the immunological synapse. These findings suggest that PAR1 signaling is involved in several effector functions of CD8 T cells including the cytotoxicity by the granule exocytosis pathway. These data have implications in CD8 T cell function and disease.